1. Field of the Invention
The present invention relates generally to respiratory gas analysis and, more particularly, to a lightweight, small size, self-contained metabolic rate transducer capable of being carried by a facemask of a subject at rest or undergoing physical activity or incorporated in a respiratory circuit.
2. Discussion of the Prior Art
All of the processes taking place in the body ultimately result in the production of heat. Heat production and caloric consumption or metabolism can be viewed in a similar context. Indirect calorimetry is a practical means by which heat production is measured to quantify metabolic rate or function.
All energy production or metabolism in the body ultimately depends on the utilization of oxygen. Indirect calorimetry involves directly measuring the consumption of oxygen (O2) and the production of carbon dioxide (CO2) through quantitative analysis of inspired and expired air flow, oxygen, and carbon dioxide to provide an accurate measure of energy metabolism. Energy production or metabolism measurement through indirect calorimetry at rest and during activity is used by physicians for clinical reasons, by coaches to measure athletic performance, and by trainers to measure fitness levels. There are many different gas analysis techniques used in the prior art of indirect calorimetric, some of which only estimate oxygen consumption since they lack the ability to directly measure carbon dioxide production which is necessary to account for the difference in inhaled and exhaled air flow of the subject. This compromised approach, many times an attempt to reduce size, weight, cost, and complexity of the apparatus or to circumvent the challenges of gas transport from the subject to the sensor itself results in poor accuracy and less repeatable measures of metabolism. Further, these abbreviated methods fail to account for intersubject variations because they rely on assumptions made from population averages.
It has been known for some time the analysis of a subject's respiration provides valuable information relating to the physical condition of the subject. The four most commonly measured variables are: (1) respiratory volume; (2) oxygen consumption; (3) carbon dioxide production; and (4) respiratory exchange ratio (RER), which is the ratio of carbon dioxide produced to oxygen consumed. One of the earliest efforts to conduct indirect metabolic rate analysis involved the use of a so-called Douglas Bag. A Douglas Bag metabolic analysis technique involved the timed collection of expired breath in a rubberized bag, measuring the volume of expired gas collected and analyzing the gas composition contained within the rubberized bag for O2 and CO2 content. Metabolic rates were then calculated from the data obtained. The Douglas Bag technique was time consuming, subject to error and could only be performed on relatively stationary subjects in well-equipped laboratories. Also, this technique was not well-suited to the measurement of short-duration transients in metabolic functions.
Since the data obtained from respiratory gas analysis is so valuable in diagnosing cardiopulmonary dysfunction and evaluating overall cardiovascular fitness, intense effort has been directed towards the development of simpler and faster automated metabolic analyzers. The intense interest in physical fitness and aerobic exercise, such as running, has helped to focus further effort in this field. Various instruments are presently available for the determination of the total volume of respired air from a subject being studied. These devices include spirometers, plethysmographs, and pneumotachographs. Numerous instruments are also available for determining O2 and CO2 content in respired gas. Some of the more recent techniques involve the use of a discrete zirconium oxide (ZrO2) sensor and a non-dispersive infrared (NDIR) gas analyzer for determining CO2 content. A metabolic analyzer of the type described is disclosed in U.S. Pat. No. 4,463,764 to Anderson, et al. While such instruments are accurate, they are large, heavy, and require frequent calibration as well as special operating skills. An instrument described in the Anderson et al. patent is so large a fixed equipment rack incorporates all of it and it can only be used in a clinical or laboratory setting.
U.S. Pat. No. 5,363,857 to Howard describes a metabolic rate analyzer having a CO2 detector, an O2 detector, a flow resistance, a differential pressure transducer, a solenoid-actuated metering valve for producing a volumetrically-proportional sample of respired gas, a vacuum regulator for receiving the sample, a pump for drawing the sample from the vacuum regulator and a processor for periodically sampling the differential pressure signal to provide a flow signal, to modulate power applied to the solenoid-actuated flow proportioning valve and to provide a measure of the total volume of respired gas. The processor in the Howard analyzer is programmed to correlate the total volume of respired gas, O2 content, and CO2 content to provide a measure of metabolic rate.
While the apparatus described in the Howard '857 patent is of a reduced size when compared to the earlier Anderson equipment and can be worn on the body for ambulatory applications is not miniaturized to the point where it can be an integral part of a facemask assembly, as well as not being able to provide true breath by breath analysis. Adding to the weight and bulk of the Howard apparatus is the reliance upon solenoid-operated metering valves and an electromechanical pump for moving respiratory gas through O2 and CO2 analyzers.
U.S. Pat. No. 6,955,650 to Mault et al. describes a portable device for measuring the metabolic rate of an individual including a respiratory gas flow path containing a hygiene barrier capable of blocking a predetermined pathogen possibly present in exhaled gases. The flow pathway is contained within the interior of an outer housing and includes a flow tube leading to a flow meter and an oxygen sensor. The device further includes a “computation unit” utilizing the outputs from the flow meter and the oxygen sensor to determine metabolic rate. Carbon dioxide production is computed rather than measured and this can lead to significant inaccuracies in establishing the true metabolic rate of the subject and the true substrate utilization.
While the Mault et al. '650 patent indicates a CO2 sensor may be incorporated into the device so as to directly measure, rather than calculate, CO2 concentrations, it fails to teach how such a device can be configured so as to be sufficiently small and lightweight to be incorporated into a metabolic analyzer able to be supported on a subject's facemask.
It is accordingly one aspect of the present invention to provide novel, non-invasive, lightweight, small size metabolic analyzers that can either be carried by a facemask worn by a subject or incorporated in a respiratory circuit and accurately providing output signals corresponding to a subject's metabolic rate and respiratory exchange ratio and gas concentrations on a real-time, breath-by-breath basis.
Still another aspect of the invention is to provide a metabolic analyzer incorporating a removable, lightweight optical guide incorporating both an open channel mainstreamed sample chamber of CO2 detection and an orifice plate for establishing a pressure drop and subsequent flow analysis using a differential pressure transducer.
Another aspect of the invention is to provide a new and improved metabolic analyzer weighing in the range of 10 to 3 oz. and possibly less than 5 oz.
Another aspect of the invention is to provide a metabolic analyzer especially constructed for use in a health club setting or for personal use for providing basic metabolic information on which a work-out prescription can be structured either for optimizing weight loss (fat burning) or cardiorespiratory conditioning.